Plant protein and body weight in postmenopausal women: a secondary analysis from a randomized clinical trial

OBJECTIVE: This secondary analysis evaluated whether changes in plant and animal protein intake were associated with changes in body weight among postmenopausal women participating in a 12-week randomized dietary intervention. METHODS: A secondary analysis of a 12-week randomized clinical trial compared the effects of a vegan diet supplemented with 86 g cooked soybeans (n=42) to a usual diet (n=42) in postmenopausal women. An analysis of 3-day diet records examined animal and plant protein intake as predictors of body weight changes. Effect size was defined as the between-group difference (vegan minus control) for the 12-week change. RESULTS: Total protein intake did not change in either group. Animal protein decreased in the vegan group by 23.3 g (95% confidence interval [CI]: -31.3 to -15.4), with no change in the control group; effect size: -24.5 g (95% CI: -33.9 to -15.1; P<0.001). Plant protein increased in the vegan group by 22.1 g (95% CI: 17.4 to 26.8), with no change in the control group; effect size: 23.8 g (95% CI: 16.3 to 31.3; P<0.001). Changes in animal protein correlated positively with changes in body mass index (r=0.59; P<0.001), while changes in plant protein correlated negatively with changes in body mass index (r=-0.51; P<0.001), even after adjustment for energy intake changes (r=0.53; P<0.001; and r=-0.54; P<0.001, respectively). A 1-kg of weight loss was associated with a reduction in animal protein intake by 16.2 g/day (P<0.001) and an increase in plant protein intake by 12.7 g/day (P<0.001). Methionine intake decreased in the vegan group by 0.37 g (95% CI: -0.56 to -0.18), with no change in the control group; effect size: -0.41 g (95% CI: -0.66 to -0.15; P=0.002). CONCLUSIONS: Replacing animal protein with plant protein was associated with decreased body weight, independent of changes in energy intake. A dose-response relationship was observed between reducing animal protein intake and increasing plant protein consumption, and weight loss.